Downhole tools use slips to engage a casing and hold a downhole tool in place. For example, packers are a type of downhole tool that uses slips. Packers are used in oil and gas wells primarily to isolate different production zones. On the packer, a slip provides a frictional hold between the packer and casing or wellbore that helps keep the packer in place when subjected to high pressure, high temperature, and applied forces. The packer and associated slip is either permanent or retrievable.
Permanent packers are usually less expensive to manufacture and are typically capable of withstanding high pressure and temperature. In contrast, a retrievable packer can be “unset” by using hydraulic or mechanical means. After the packer is “unset,” it can be brought uphole with tubing or a work string. Because it is designed to be reusable, a retrievable packer is, typically, more complex and has more mechanical parts.
Because it is permanent, a permanent packer is typically destroyed by milling or drilling to remove it. In other words, the permanent packer is designed for a single use and is destroyed to remove it. Thus, it is desirable to construct a permanent packer from materials that are more readily milled or drilled. Examples of materials that are more readily milled or drilled are made from non-metallic materials, such as composites, ceramics and plastics. Plastics such as ultra-high-molecular-weight polyethylene (UHMW), polytetrafluroethylene (PTFE) or similar engineering grade plastics can be used because of their high molecular weight and long molecular chains, although other thermoplastic polyethylenes might also be used.
Broadly speaking, more readily milled/drilled materials are weaker and are therefore less capable of carrying a load. Correspondingly, forming a permanent packer from more sturdy metallic materials makes the permanent packer stronger. However, the added strength means that it is more difficult to mill or drill the permanent packer to remove it. Added strength to the packer means that additional rig time is required to mill or drill the packer to remove it. Thus, there is an inherent contradiction between using permanent packers composed of metallic materials because it is significantly more time consuming to mill or drill when they are no longer needed. Because rig time is expensive, the added expense of additional rig time can equal or exceed the savings of using a permanent packer as opposed to a retrievable packer.
The use of more durable metallic materials can also cause a problem known as “bit tracking” to occur when a drilling or milling a metallic material. During bit tracking, the drill bit used to mill out the tool stays on one path and no longer cuts the material to be drilled or milled. When this happens, it is appropriate to pick up the bit and rapidly recontact the material being drilled. During bit tracking, some material may be removed, but in actuality the drill bit is merely wearing against the surface of the downhole tool. Essentially, during bit tracking, the drill bit is rotating, but it is not appropriately cutting the packer or other material to be removed. Unfortunately, it might not be readily apparent to operators at the surface that bit tracking is occurring because the drill bit continues to rotate normally, even though it is not drilling or milling the packer or other material to be drilled.
A downhole tool may be used when it is desirable to seal tubing or other pipe in the casing or wellbore of the well, such as when it is desired to pump cement or other slurry out into a formation. In this situation, it is appropriate to seal the tubing with respect to the well casing and to prevent the fluid pressure of the slurry from lifting the tubing out of the well. Packers, bridge plugs, and the like are designed for these general purposes. Slip mechanisms are devices used on these downhole tools to contact the wellbore and hold the downhole tool in the wellbore without substantial movement, and as discussed above, to hold back fluid or pressure. Typically, the slip mechanism is used to contact the wellbore to hold the downhole tool in the wellbore without substantial movement.
Current slip mechanisms used in the art include T-style, hydro-style, and arrow-style slip mechanisms, which are briefly discussed.
FIGS. 1A-1B show a T-style slip mechanism 10 according to the prior art. The mechanism 10 includes several slip components, such as T-style slips 20, a cone 30, and a cage 40—each of which dispose on a mandrel 14 of a packer 12 or the like. The T-style slips 20 have wickered ends 24 and T-shaped ends 28 interconnected by necks 22. Slip slots 42 in the cage 40 hold the T-shaped ends 28, while slots 32 in the cone 30 hold the wickered ends 24. In particular, the wickered ends 24 have shoulders or ledges 25 (FIG. 1A) that ride in grooves 35 in the cage's slots 32.
The T-style slips 20 set into the casing wall when the cone 30 is mechanically or hydraulically moved closer to the slip cage 40. For this reason, the slips' wickered ends 24 have ramped edges 27 that are pushed by the cone 30.
FIGS. 2A-2B show a hydro-style slip mechanism 110 according to the prior art. The mechanism 110 includes several slip components, such as hydro-style slips 120, a cone 130, and a cage 140—each of which dispose on a mandrel 14 of a packer 12 or the like. The hydro-style slips 120 fit around the mandrel 14 and have wickered faces 124a-b that fit through slip slots 142 in the cage 140. A spring 160 disposes in a central passage 122 along the length of the slip 120 and sits beneath a central band 144 in the slip slots 142. This spring, which is usually a leaf style spring, biases the slip 120 to a retracted condition when the cone 130 has been pulled out of the set position. As shown in the set position, however, the hydro-style slip 120 has wickers 126 on its outer face that can set into the surrounding casing wall (not shown).
To set the hydro-style slip 120 into the casing wall, the cone 130 is moved (typically by hydraulic activation) further beneath the slip cage 140 and also beneath the hydro-style slips 120. A ramped edge 137 on the cone 130 pushes against the ramped end 127 of the slip 120. Therefore, the cone 130 should slide beneath the slip cage 140 to push the slips 120 through the slip slots 142.
FIGS. 3A-3B show an arrow-style slip mechanism 210 according to the prior art. This mechanism 210 includes several slip components, such as arrow-style slips 220, a cone 230, and a cage 240—each of which dispose on the mandrel 14 of a packer 12 or the like. The arrow-style slips 220 fit around the mandrel 14 and have wickered ends 224 and fitted ends 228 interconnected by necks 222. The fitted ends 228 fit in comparably shaped slots 242 in the cage 240, while the necks 222 fit under a shoulder area 244 on the edge of the cage 240. The arrow-style slip 220 sets into the casing wall when the cone 230 is mechanically or hydraulically moved closer to the slip cage 240. In particular, the wickered end 224 of the slip 220 includes a ramped edge 227 on its inner side. When the cone 230 is moved toward the cage 240, the cones ramped edge 237 engages the slip's ramped ends 227, pushing the slip's wickered end 224 into the casing wall. When the slip 220 sets, the wickers 226 on the slip's wickered end 224 set into the surrounding casing wall (not shown). Whether the slips 220 are set or not, the cage 240 remains connected to the fitted ends 228 of the arrow-style slip 222 by virtue of these slip slots 242.
The requirements for slips are that they bite or lock in a tool; the prime example being a packer slip used to lock the packer in a selected position in casing or wellbore. The problem is to make the slips easier to remove by milling or drilling techniques thereby cutting well construction, completion time, and costs.
The prior art slips have been made from gray and ductile types of cast irons. These cast irons are more readily millable/drillable, but still require significant milling/drilling time. More recently, slips have been made with ceramic biting elements glued in composite slip bases. The work in composite slips is promising but unproven because there may be ductility issues with the composite slip base materials. Thus, these solutions, at this point, have provided less than an ideal solution.
In addition, it is known to harden the surface of an aluminum metallic packer by anodizing the surface to form an anodized metallic coating. However, this is problematic because anodization has been found to produce very thin coatings of only a few angstroms or microns. Because this is a relatively thin layer, the slip cannot readily adhere with the substrate. Ion sputtering has also been used to form a metallic coating. However, this also produces only a thin substrate on the order of a few angstroms or microns, which has also been found to lack sufficient strength.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.